Bug 163978

Summary: [hwpmc] [patch] Loading hwpmc with an unknown cpuid causes a page fault in kernel mode.
Product: Base System Reporter: Alan Somers <alans>
Component: kernAssignee: freebsd-bugs (Nobody) <bugs>
Status: Open ---    
Severity: Affects Only Me CC: nicolas.masse
Priority: Normal    
Version: 10.0-CURRENT   
Hardware: Any   
OS: Any   

Description Alan Somers 2012-01-09 23:20:12 UTC
Attemping to use hwpmc from a Xen guest on a Core i7 running in 64 bit mode, I got the following crash on bootup:

xbd0: 20480MB <Virtual Block Device> at device/vbd/768 on xenbusb_front0
xbd0: attaching as ad0
Timecounter "TSC" frequency 2000036316 Hz quality 800

Fatal trap 12: page fault while in kernel mode
cpuid = 0; apic id = 00
fault virtual address	= 0x0
fault code		= supervisor read instruction, page not present
instruction pointer	= 0x20:0x0
stack pointer	        = 0x28:0xffffffff81985b90
frame pointer	        = 0x28:0xffffffff81985c10
code segment		= base rx0, limit 0xfffff, type 0x1b
			= DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags	= interrupt enabled, resume, IOPL = 0
current process		= 0 (swapper)
[ thread pid 0 tid 100000 ]
Stopped at      0:      *** error reading from address 0 ***
db> where
Tracing pid 0 tid 100000 td 0xffffffff8154f8b0
uart_sab82532_class() at 0
load() at load+0x39
syscall_module_handler() at syscall_module_handler+0xe6
module_register_init() at module_register_init+0xb0
mi_startup() at mi_startup+0x139
btext() at btext+0x2c

The problem was that Xen changes the cpuid that guests see.  Specifically, the result of do_cpuid(CORE_CPUID_REQUEST, cpuid) is changed to indicate that performance counters are not supported.  Hence, pmc_core_initialize() returns EPROGMISMATCH.  Eventually, that causes pmc_initialize() to execute a null function pointer at line 4692.  Really, the root cause of the problem is that hwpmc isn't written very defensively.  I've attached a patch that makes the panic go away, but doesn't make PMC work for my processor.


==== sys/dev/hwpmc/hwpmc_mod.c#2 (text) ====

@@ -4689,7 +4689,15 @@
                if (md->pmd_pcpu_init)
                        error = md->pmd_pcpu_init(md, cpu);
                for (n = 0; error == 0 && n < md->pmd_nclass; n++)
-                       error = md->pmd_classdep[n].pcd_pcpu_init(md, cpu);
+                       if (md->pmd_classdep[n].pcd_pcpu_init != NULL)
+                               error = md->pmd_classdep[n].pcd_pcpu_init(md,
+                                   cpu);
+                       else
+                               printf("hwpmc: md->pmd_classdep[%d]."
+                                   "pcd_pcpu_init was not initialized.  "
+                                   "This is a bug.\n"
+                                   "\tMost likely your CPUID is not "
+                                   "recognized by hwpmc.\n", n);
@@ -4882,7 +4890,15 @@
                        for (c = 0; c < md->pmd_nclass; c++)
-                               md->pmd_classdep[c].pcd_pcpu_fini(md, cpu);
+                               if (md->pmd_classdep[c].pcd_pcpu_fini != NULL)
+                                       md->pmd_classdep[c].pcd_pcpu_fini(md,
+                                           cpu);
+                               else
+                                       printf("hwpmc: md->pmd_classdep[%d]."
+                                           "pcd_pcpu_fini was not initialized."
+                                           "  This is a bug.\n"
+                                           "\tMost likely your CPUID is not "
+                                           "recognized by hwpmc.\n", c);
                        if (md->pmd_pcpu_fini)
                                md->pmd_pcpu_fini(md, cpu);
How-To-Repeat: Compile an amd64 Xen domU kernel with the below lines in your configuration file and boot it:
options    HWPMC_HOOKS
device     hwpmc
Comment 1 Davide Italiano freebsd_committer 2013-05-02 14:16:37 UTC
State Changed
From-To: open->feedback

Alan, some changes have been recently committed to HEAD, and the problem 
should be solved. I've not tested on Xen because I've not a machine with 
that available, but I was able to reproduce this same bug on VirtualBB. 
Do you mind to test if it also fixes the problem for you?
Comment 2 asomers 2013-05-03 21:49:55 UTC
Actually, I don't have a Xen machine anymore, either.  Sorry.
Comment 3 Eitan Adler freebsd_committer freebsd_triage 2017-12-31 08:01:28 UTC
For bugs matching the following criteria:

Status: In Progress Changed: (is less than) 2014-06-01

Reset to default assignee and clear in-progress tags.

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